Wednesday, July 20, 2011

When it comes to hops, brewers are mostly interested in two things: alpha acids and essential oils. In a nutshell, alpha acids provide bitterness while essential oils provide flavor and aroma. That being the case, I'm surprised by how many brewers account for seasonal variability by keeping the alpha acid contributions of each kettle addition the same from batch to batch. A better way to achieve consistent hoppiness, especially for beers where hop flavor and aroma are important, is to keep the total oil contributions of each hop addition the same from batch to batch - except for the primary bittering addition - and adjust the primary bittering addition to maintain the overall IBU level.

Suppose a brewer makes a great 5-gallon batch of IPA with the following hop bill:

The following year, the brewer wants to reproduce the batch. However, the new crop of Magnum are 12% alpha acids and the new crop of Centennial are 10% alpha acids. If the brewer tries to maintain alpha acid consistency instead of total oil consistency, the hop bill will look like this:

If the oil content of the Centennials remains constant, the beer will have fewer hop oils than the previous batch. In other words, the new batch won't be as hoppy. Unfortunately, hop brokers don't often provide information on total oil content. Commercial brewers can ask for it (although it seems that few do), but homebrewers have less leverage because they don't buy directly from brokers. If you don't have oil data, you can assume the oil levels of a given variety will stay the same from purchase to purchase. Back to our example, maintaining the Centennial oil contributions of the original batch and adjusting the Magnums to compensate for IBU fluctuations will result in the following hop bill:

Note that the weights of Centennials are the same as the initial batch. That's the result of our assumption that oil levels remain constant. Keeping the weight of each late hop addition the same works well as long as you don't switch varieties. However, what if you want to use Cascade instead of Centennial but still keep the hop "intensity" the same? A good resource is the HopUnion Hop Variety Data Booklet, which you can download here. You can see that the middle value for total oil content of Centennials is 1.9 mL/100 g and the corresponding value for Cascades is 1.15 mL/100 g. To adjust the hop bill, multiply the Centennial weights by 1.9 and divide by 1.15 to end up with the following values:

3 comments:

Very good points, which i've never considered - mainly because (as you said) oil levels aren't readily provided to us homebrewers.

Another point to back your argument about batch-to-batch consistency is that you'd be hard pressed to find someone who's got a sensitive enough palate to detect (say, from your example) a 4IBU difference at the flavor addition of 20min. in two beers. So matching the oil levels instead of the exact IBU values might be a better route for the flavor & aroma additions.